VO2 is a fascinating correlated-oxide material that possesses strong coupling among its charge, spin, orbital, and lattice degrees of freedom. VO2 exhibits a sharp metal-insulator transition (MIT) above room temperature (i.e., transition temperature TMIT of ˜341 K in bulk) with an accompanying structural-phase transition from high-temperature rutile to low-temperature monoclinic structures. This unique property coupled with an almost five-orders-of-magnitude conductivity change (in single-crystal bulks) across the transition make VO2 a compelling model system for scientific and technological endeavors. Furthermore, the ultrafast nature of VO2's MIT gives it diverse potential applications in materials physics and solid-state electronics. Critical to any practical application for VO2, as well as to exploration of its fundamental physics, is the ability to grow high-quality epitaxial thin films.
Yet it has been difficult to achieve heteroepitaxy in VO2 thin films due to several intrinsic problems that hamper reliable and predictable VO2 device performance. Genuine epitaxial growth without rotational domain variants has been achieved with a TiO2 substrate, owing to the rutile, isostructural symmetry between VO2 and TiO2 at their respective growth temperatures. Despite structural compatibility, though, there is a slight lattice mismatch of ˜1.0% between VO2 and TiO2, causing a gradual strain relaxation when a film's thickness exceeds a critical value (i.e., ˜20 nm), and this results in severe inhomogeneities throughout the films and in a broad MIT. Even worse, this strain relaxation is accompanied by the formation of cracks that degrade VO2's MIT features, including its magnitude of resistance change across the MIT.